Paul Hobson reveals the threat posed by the invasion of North American signal crayfish to the white-clawed crayfish native to our rivers

When I was a young lad I became fascinated by the dales of the White Peak. During one spring holiday I told my parents I was going to cycle to Beresford Dale with a couple of my friends and camp out for a few days. All credit to them, they didn’t bat an eyelid!

During one of our rambles along the river I came across a lot of shells of eaten crayfish and that evening, by torchlight, we watched a number of crayfish exploring the river bed. These were our native, white-clawed crayfish and the rivers of Cheshire, Staffordshire and Derbyshire in the 1970s held healthy populations of these fantastic crustaceans. Roll on 40 years and the situation is now very different.

In the 1970s farming was experiencing a mini-crisis (when isn’t it?) and the government was encouraging farmers to diversify and try out new ideas of ways to earn extra cash. One of these was crayfish farming. The crayfish market was very lucrative then, particularly in Scandinavia. What was wanted was a crayfish species that grew fast and big to maximise profits.

In the Victorian period Britain, as well as Germany, exported native crayfish to France. T H Huxley in his monograph of The Crayfish in 1879 wrote: ‘In this country we do not set much store upon crayfish as an article of food, but on the continent, and especially in France, they are in great request. Paris alone, with its two millions of inhabitants consumes annually from five to six millions of Crayfishes, and pays about £16,000 (£1.8 million today) for them.’ He went on to add that crayfish farming was a successful business in the Victorian era.

Signal Crayfish, Pacifastacus leniusculus

However, in the 1970s our native white-clawed crayfish was considered too small and slow-growing to be economically viable, so an alternative was sought. In came the huge signal crayfish of North America – much larger, much more aggressive and faster growing. (This somewhat reminds me of the grey versus red squirrel debate.) The inevitable happened – some escaped and some were deliberately released into British rivers and ponds. The red-clawed nightmare had begun!

Most signal crayfish farms were situated in the South of England and from here the signals rapidly made their way through the river systems, allowing them to spread north. In the past white-clawed crayfish had been recorded in all the major rivers of Derbyshire and the Peak. Today the situation is dire. There are still small populations here and there but in most of our major rivers such as the Lathkill they have been wiped out. So, the big question is, why have our natives been so heavily depleted?

Signals are large and aggressive and they simply outcompete our much smaller native crayfish. This in itself is bad enough but there is a deadly double whammy – signals are immune to crayfish plague, a fungal disease which they can carry and which is lethal to our native species. Infected signals release spores into the water, which effectively spreads the disease downstream. Boots, wellies, dog fur and fishing tackle can all pick up the spores from infected water, and these spores can stay alive in damp conditions for two weeks. Humans can easily become the disease vectors.

The crayfish plague is bad enough but the damage signals can cause doesn’t stop there. During winter signals like to avoid the cold by lying in deep burrows which they dig into muddy banks. These banks can collapse creating danger for livestock. The burrowing releases sediment into the river which subtly changes its biology and reduces invertebrates – the food of fish and birds like dippers. Signals are voracious feeders and will happily eat invertebrates such as caddis fly larvae, tadpoles of frogs and newts, fish eggs and, of course, our native white-clawed crayfish. In some cases they can even displace water voles from their burrows, arguably Derbyshire’s and Britain’s fastest declining mammal. However, signals don’t always get it all their own way. They themselves are prey to otters, mink and herons, and the small ones are eaten by dippers and larger predatory fish like salmon and trout. The major point, however, is that when signals move in we lose our white-claws.

Signal Crayfish, Pacifastacus leniusculus

One solution that has been suggested is to increase our fishing of them. (A licence is required to fish for them.) Signals are good to eat and the new craze of wild foraging, together with the fact that they have featured in the recipes of some TV celebrity chefs, has seen more people fishing for them. On the face of it this seems a win-win situation, but it is not so. Fishing for signals targets the biggest ones but an effect of catching all the larger ones is to remove one of the limits on the population growth of their smaller brothers and sisters. In many cases this results in a population boom – now a lose-lose situation. There is also some evidence that some folk who want to add them to their diet have illegally spread them to new areas.

The rivers of the Peak District may have taken a real pounding from signals but all is not lost. Next month I will explore how local organisations and groups of crayfish enthusiasts are fighting back in the war against the invading American.